1. Field of the Invention
The present invention relates to an isolated operation detecting method for detecting whether a distributed power supply is isolatedly operated from an electric system or not on the basis of a power fluctuation injected to the electric system, a control apparatus for detecting isolated operation of a distributed power supply, an isolated operation detecting apparatus, and a distributed power supply.
2. Description of the related arts
An isolated operation refers to a condition in which a distributed power supply supplies electric power to a local load of the system when an electric system stops its operation due to an accident or the like. The continuation of the isolated operation of the distributed power supply greatly affects the safety of human bodies or facilities.
A distributed power supply is installed to a customer place or its neighborhood for generating power. The current mainstream of such distributed power supply includes a cogeneration system using gas turbine, gas engine, solar power, and the like. In addition to these systems, it is expected for the future that a power generation system using renewable energy such as wind power, small-scale hydraulic power, biomass, or the like, or waste materials or the like; micro gas turbine that is technically under developing; or a fuel cell will be spread. Of these, a fuel cell is highly expected to be the mainstream of the distributed power supply, and is considered to be installed not only to a large-scale facility such as a factory but also to a small-scale facility such as ordinary house.
Typical compact distributed power supplies include gas engine, gas turbine, micro gas turbine, solar power, fuel cell, or the like. An electric power company has made an enormous capital investment to build a power station at a place far away from a customer place, and has made a power transmission to the customer place. Such long distance power transmission produces power loss, causing power generation efficiency to be approximately as high as 30%.
In an isolated operation detecting apparatus for detecting an isolated operation for the distributed power supply explained above, various power fluctuation systems such as reactive power fluctuation system, active power fluctuation system, harmonic injection system, or the like have already been proposed as a practical detecting system of the isolated, operation, although the detailed explanation is omitted.
In the detection of the isolated operation in the power fluctuation system described above, it takes 0.5 to 1.0 seconds as isolated operation detecting time to detect the isolated operation when it occurs. However, high-speed detecting technique for shortening this isolated operation detecting time to not more than 0.1 second has been demanded.
FIG. 25 is an imaginary diagram in which multiple distributed power supplies are interconnected. The isolated operation detecting time of a power line conditioner is 0.5 to 1.0 second in the active system. This is a characteristic assuming the isolated operation (i) per each house, and the detecting time was acceptable when distributed power supplies were not widely used. However, as distributed power supplies have become more popular in these days, multiple distributed power supplies have been interconnected as shown in FIG. 25. In this case, the isolated operation may occur (ii) per pole-mounted transformer, (iii) per sectionalizing switch, or (iv) per breaker. When these high-voltage devices are included in isolated operation detecting system, a high-speed detection of the isolated operation is needed assuming an accident caused by contacting high-voltage section and low-voltage section.
There are many techniques relating to the isolated operation. The representative examples relating to the isolated operation are disclosed in Japanese Patent Application Laid-Open No. 02(1990)-144615, Japanese Patent Application Laid-Open No. 08(1996)-98411, Japanese Patent No. 3397912, and Japanese Patent No. 3424443.
However, the subject in the above-mentioned high-speed detection is to prevent misdetection (unnecessary operation) that is erroneously detecting an isolated operation when no isolated operation occurs as explained below. This unnecessary operation is caused by various factors. It has been technically difficult to realize the high-speed detection as well as to prevent the unnecessary operation.
The reason is as follows. Firstly, when current flows through a load, or power-generating current flows from an inverter in a distributed power supply, regional voltage is produced by a resistance component or inductance component of a power line. This voltage fluctuates by a load fluctuation or fluctuation in the output from the distributed power supply, which causes a voltage vector at an incoming terminal to change, whereby seemingly causing a system phase, or a system cycle to be changed. On the other hand, in the power fluctuation system, as a power fluctuation element becomes conspicuous upon an isolated operation, and the isolated operation is detected from the change in the system cycle produced by the conspicuous power fluctuation component, it is difficult to distinguish whether the change in the system cycle is produced by the resistance component or inductance component in the power line or by the isolated operation. Therefore, this often leads to unnecessary operation of erroneously detecting the isolated operation even when no isolated operation occurs.